Method of fabricating graded media

ABSTRACT

A magnetic data storage medium comprising: an ion doped magnetic recording layer having a continuous grading of coercivity or anisotropy, wherein the coercivity or anisotropy is at a minimum substantially at one side of the magnetic recording layer, and having substantial portion of maximum coercivity or anisotropy at the other side of the magnetic recording layer. Also, a method of fabricating a magnetic data storage medium is included.

FIELD OF THE INVENTION

This invention relates with manufacturing of magnetic recording mediafor hard disk drives with low switching field and high density storagecapabilities.

BACKGROUND

A magnetic recording device such as hard disk drive would have thefollowing key components: a recording medium to store information, awriting head to produce localized magnetic fields for writinginformation and a read sensor to convert the magnetic field from themedia to electrical signals.

Each magnetic bit in the current perpendicular magnetic recording (PMR)medium comprises several thermally stable magnetic grains. Furtherincrease in the areal density requires a reduction in the grain size toretain the signal-to-noise ratio.

However, the thermal stability factor (K_(u)V/k_(B)T, where K_(u) is themagnetocrystalline anisotropy, V the magnetic grain volume, k_(B) theBoltzmann constant and T the absolute temperature) of each magneticgrain should be >60.^(1,2) Therefore, the reduction in the magneticgrain size reduces the thermal stability as a result ofsuper-paramagnetism.³ Super-paramagnetism is a phenomenon by virtue ofwhich magnetization direction of smaller magnetic particle switcheswithout any applied magnetic field due to ambient thermal energy. Inthis case thermal energy (k_(B)T), becomes comparable to the anisotropyenergy (K_(u)V) and magnetizations thermally flip the direction, whichundesirably causes random data corruption. Putting another way, themagnetic grains lose the data undesirably without any applied field.Thus decreasing grain size is not by itself a solution to increasingareal density.

An alternative to delay the superparamagnetism is by using a materialwith large in order to keep the magnetic grains thermally stable, andhence the thermally stable bits. Prior art writing heads currently havea limitation of the maximum writing field of 24 kOe, and are unable toswitch high K_(u) mediums such as CoPt and FePt, since switching fieldis proportional to magnetocrystalline anisotropy. One of the keychallenges for the realization of high K_(u) materials based media forindustrial application is to reduce the switching field.⁴

Exchange coupled composite (ECC) bilayer media⁵ or exchange springmedia⁶ (independently proposed by Victora et al. and Suess et al.,respectively) may be effective at reducing the switching field. Suchmedium consist of magnetically hard and soft regions within eachmagnetic grain, where soft region assists hard region to reduce theswitching field by formation of domain wall at the opposite end of thehard\soft interface followed by domain wall propagation towards theinterface (FIG. 1.). In addition to reducing the writing field, ECCmedia also enjoys other advantages over conventional perpendicularmagnetic recording such as faster switching and insensitivity to widerrange of easy axis distribution.

Furthermore it has been theoretically predicted and experimentallyobserved that multilayer media or “graded media” in which anisotropyvaries substantially continuously along the film growth direction, maybe more effective for switching field reduction than the bilayer ECCmedia.^(7,8) However, preparing graded media is extremely challengingtask using conventional sputtering technique. An approach to fabricateFe/FePt graded media has been reported where Fe was deposited at hightemperature on FePt, and composite film was annealed at hightemperature.⁹ In this case diffusion of Fe into FePt may produce gradedmedia. However, major drawback of this technique is that the control ofFe diffusion is very difficult. Moreover, annealing may also inducegrain growth in the lateral direction, which undesirably deterioratesthe signal-to-noise ratio.

Another approach being used is introducing an impurity during sputteringinto the layer to increase the magnetic softness of hard magneticmaterial. But this way there are different layers formed with reducedimpurity content hence it is more of a multilayer medium rather than acontinuously graded media.¹⁰

SUMMARY

In general terms the invention uses ion-implantation to achieve thegraded media. The ions may be implanted into the recording layer in sucha manner that the doping profile has its peak at the top surface ofrecording layer and a gradual tail goes deep into the recording layerhence creating a gradient in anisotropy across the recording layer.

In a first specific expression of the invention there is provided amagnetic data storage medium comprising: an ion doped magnetic recordinglayer having a continuous grading of coercivity or anisotropy, whereinthe coercivity or anisotropy is at a minimum substantially at one sideof the magnetic recording layer, and having substantial portion ofmaximum coercivity or anisotropy at the other side of the magneticrecording layer. The magnetic recording layer may be a highmagnetocrystalline anisotropy material. The high magnetocrystallineanisotropy material may be selected from the group consisting or CoPt,FePt, SmCo₅ and any other magnetic materials comprising uniaxialmagnetic anisotropy >1×10⁷ ergs/cc.

The continuous grading of coercivity or anisotropy may have a relativelysharp gradient profile at the interface of the overcoat and therecording layer, wherein an ion density is at a maximum substantially atthe interface and the coercivity is a minimum substantially at theinterface.

The ions may be selected from the group consisting of helium (He⁺),carbon (C⁺), nitrogen (N²⁺), argon (Ar⁺), cobalt (Co⁺), antimony (Sb⁺)and any combination thereof.

The ions may be cobalt (Co⁺).

The fluence of the ions may range from 10¹⁴ to 5×10¹⁶ ions/cm².

In a second specific expression of the invention there is provided amethod of fabricating a magnetic data storage medium comprising:providing a magnetic recording layer, and implanting ions into themagnetic recording layer to provide a continuous graded level of iondoping within the magnetic recording layer.

The step of implanting ions may comprise selecting a species of ions, anangle of implantation and an energy of implantation to achieve a maximumdoping level substantially at one side of magnetic recording layer andhaving substantially undoped portion at the other side

The species of ions may be selected from the group consisting of helium(He⁺), carbon (C⁺), nitrogen (N²⁺), argon (Ar⁺), cobalt (Co⁺), antimony(Sb⁺) and any combination thereof.

The method may further comprise selecting a fluence of the ions frombetween 10¹⁴ to 5×10¹⁶ ions/cm².

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be fully understood and readily put intopractical effect there shall now be described by way of non-limitativeexample only, example embodiments described below with reference to theaccompanying illustrative drawings in which:

FIG. 1 is a schematic diagram of a field reversal in an ECC media usinga spin chain model;

FIG. 2 (a) is a graph of an ion implantation profile of carbon at 5.8keV in the recording layer according to an example embodiment of thepresent invention; FIG. 2( b) is a graph of an ion implantation profilein a complete media structure;

FIG. 3 is a schematic diagram of graded media by ion implantation; and

FIG. 4 is a graph of switching field verses dose for different implantedspecies.

DETAILED DESCRIPTION

According to the example embodiment it is proposed to fabricate a gradedmedia using an ion-implantation method, which is described as follows.

-   1) Granular media of CoCrPt and FePt are deposited on Ru and CrRu    underlayer respectively. The magnetic recording layer is a high    magnetocrystalline anisotropy material and may comprise CoPt, SmCo₅,    or any other magnetic materials having uniaxial magnetic anisotropy    >1×10⁷ ergs/cc.-   2) Ru and CrRu underlayer were chosen for high anisotropy CoCrPt and    FePt magnetic recording layer, since it will control the easy axis    of magnetization along the film normal direction required for    perpendicular magnetic recording.-   3) Magnetic recording layer is implanted with suitable ion species,    dose and energy.-   4) Ion implantation results shows uniform Gaussian doping profile    200 inside the film (FIG. 2.(a)) and helps to implant the doping    concentration in controlled way at different depth, which changes    the magnetic anisotropy in very controlled way at atomic level. The    implanted profile 202 in complete media structure is shown in    FIG. 2. (b).-   5) Ideally, the profile 300 in media should be as shown in FIG. 3,    such that a maximum doping happens on the surface of recording layer    302 and reduces as it goes below in recording layer 304. Such    continuous change in magnetic anisotropy leads to fabricate the    graded media precisely as shown in FIG. 3.-   6) Ion-implantation of Helium 400 and Carbon 402 (FIG. 4) in    addition to other ions shows that coercivity is the function of    implanted dose, energy and implanted species indicating that these    are the parameters that tailor the magnetic properties.

The ion species chosen for the graded media application shouldpreferably be able to reduce the anisotropy constant without changingthe thermal stability of grains and remanent magnetization or saturationmagnetization. Ion species which have been tried are helium (He⁺),carbon (C⁺), nitrogen (N²⁺), argon (Ar⁺), cobalt (Co⁺) and antimony(Sb⁺) starting with fluence ranging from 10¹⁴ up to 5×10¹⁶ ions/cm². Nochange in

Saturation magnetization (M_(S)) was observed for helium and cobalt,whereas, all other species showed reduction in M_(S). The coercivity oranisotropy constant reduced with increasing fluence for a given speciesand the mass of ion species. Thermal stability factor was seen to beconstant around 60 for nitrogen, oxygen and cobalt till a fluence of5×10¹⁵ ions/cm².

Ion species like cobalt may be preferred for some applications where areduced anisotropy constant without adversely affecting the saturationmagnetization and thermal stability of the grains, with increasingfluence, is desirable.

Ion implantation by adjusting the energy of the implanted ions in such away that ion implantation peak is at the top surface of recording layerand a gradual tail goes deep into the recording layer a continuousgradient of anisotropy across the recording layer can be created. Due toprecise control of the doping profile, ion implantation may be veryuniform compared to other methods which have been used in the prior artfor graded media fabrication.

Whilst there has been described in the foregoing description embodimentsof the present invention, it will be understood by those skilled in thetechnology concerned that many variations in details of design,construction and/or operation may be made without departing from scopeas claimed.

LIST OF REFERENCES

-   1. S. H. Charap, P. L. Lu and Y. He, IEEE Trans. Magn. 33, 978    (1997).-   2. A. Moser, K. Takano, D. T. Margulies, M. Albrecht, Y. Sonobe, Y.    Ikeda, S. H. Sun and E. E. Fullerton, J. Phys. D: Appl. Phys. 35,    R(157) (2002).-   3. C. P. Bean and J. D. Livingston, J. Appl. Phys. 40, 120S (1959).-   4. Rottmayer et. al. IEEE Trans. Magn. 42, 10 (2006).-   5. R. H. Victora and X. Shen, IEEE Trans. Magn. 41, 537 (2005).-   6. D. Suess, T. Schrefl, S. Faehler, M. Kirschner, G. Hrkac, F.    Dorfbauer and J. Fiedler, Appl. Phys. Lett. 87, 012504 (2005).-   7. C. Abraham and A. Aharoni, Phys. Rev. 120, 1576 (1960).-   8. D. Suess, Appl. Phys. Lett. 89, 113105 (2006).-   9. Dagmar Goll, Achim Breitling, and Sebastian Macke, IEEE Trans.    Magn., 44, 3472 (2008).-   10. C. L. Zha, R. K. Dumas, Y. Y. Fang, V. Bonanni, J. Nogués, and    Johan Akerman, Appl. Phys. Lett. 97, 182504 (2010).

The invention claimed is:
 1. A magnetic data storage medium comprising:an ion doped magnetic recording layer having a continuous grading ofcoercivity or anisotropy, wherein the coercivity or anisotropy is at aminimum substantially at one side of the magnetic recording layer, andhaving a substantial portion of maximum coercivity or anisotropy at theother side of the magnetic recording layer; and an ion-doped overcoat,wherein the continuous grading of coercivity or anisotropy has arelatively sharp gradient profile at an interface of overcoat and therecording layer, wherein an ion density is at a maximum substantially atthe interface and has a continuous grading of ion density between theovercoat and the recording layer, and wherein the coercivity is aminimum substantially at the interface.
 2. The medium in claim 1 whereinthe magnetic recording layer includes a high magnetocrystallineanisotropy material.
 3. The medium in claim 2 wherein the highmagnetocrystalline anisotropy material is selected from a groupconsisting of CoPt, FePt, SmCo₅ and magnetic materials comprisinguniaxial magnetic anisotropy >1 ×10⁷ ergs/cc.
 4. The medium in claim 1wherein the ions are selected from a group consisting of helium (He⁺),carbon (C⁺), nitrogen (N²⁺), argon (Ar⁺), cobalt (Co⁺), antimony (Sb⁺)and any combination thereof.
 5. The medium in claim 1 wherein the ionsinclude cobalt (Co⁺).
 6. The medium in claim 1 wherein a fluence of theions ranges from 10¹⁴ to 5×10 ¹⁶ ions/cm².
 7. A method of fabricating amagnetic data storage medium comprising: providing a magnetic recordinglayer, providing an overcoat layer on the recording layer, andimplanting ions into the magnetic recording layer and the overcoat layerto provide a continuous graded level of ion doping within the magneticrecording layer and across the interface between the overcoat and therecording layer.
 8. The method in claim 7 wherein the step of implantingions comprises selecting a species of ions, an angle of implantation andan energy of implantation to achieve a maximum doping levelsubstantially at one side of magnetic recording layer and having asubstantially undoped portion at the other side.
 9. The method in claim8 wherein the species of ions is selected from a group consisting ofhelium (He⁺), carbon (C⁺), nitrogen (N²⁺), argon (Ar⁺), cobalt (Co⁺),antimony (Sb⁺) and any combination thereof.
 10. The method in claim 7further comprising selecting a fluence of the ions from between 10¹⁴ to5×10¹⁶ ions/cm² .